Create a new player and write "giveall" to unlock everything or "givecash" for money.

This changes a system file; make a back-up before you proceed. Do not e-mail death threats to IGN if you mistakenly alter your FAT.

Open drive letter:\...\flatoutdir\Savegame\player001.sav with a hex editor (Notepad will not suffice for this purpose; you will need an actual hex editor program). Change the value at address 86 (hex) to FF. Save the file and enjoy the extra pocket money.